Cardiac myocyte injury occurring at reperfusion following a period of ischemia likely results in impart from intracellular calcium loading which may be due to the effects of reactive oxygen species which are generated at reperfusion. We previously established that oxygen derived free, radicals cause calcium loading and injury in isolated myocytes exposed to an exogenous hydroxyl radical generating system. The present study focuses on the mechanism of radical induced calcium loading in adult rat cardiac myocytes. During exposure to hydrogen peroxide and iron, (generates reactive hydroxyl radical) single myocytes demonstrated a transient increase in twitch amplitude following electrical field stimulation and a progressive shortening of diastolic cell length consistent with intracellular calcium loading. Aftercontractions later developed and finally the cell became inexcitable and underwent contracture. Cytosolic calcium, measured with the fluorescent probe indo-1, rose following the administration of the radical generating system. The cellular action potential, monitored with whole-cell clamp techniques, demonstrated marked progressive plateau prolongation without depolarization of the resting potential. Progressive cell shortening and twitch amplitude augmentation were prevented by stimulating the cell under voltage clamp with fixed duration depolarizations. Unstimulated cells failed to demonstrate progressive cell shortening prior to the abrupt onset of contracture. The currents underlying the prolongation of the action potential were studied. A progressive decrease in the magnitude of the calcium current was noted during radical exposure. Ik1, the inwardly rectifying k+ current, was inhibited and likely contributes to action potential prolongation, which ultimately leads to cellular calcium loading.